We synthesized a novel synthetic chemical library and used it in a forward chemical genetics screen designed to identify small molecule inhibitors of endothelial morphogenesis. This screen resulted in the identification of structurally-related small molecules that inhibited endothelial cell proliferation, migration, the ability to form tubule-like structures in matrigel, as well as neo- angiogenesis visualized using a dorsal skin fold vascular window chamber. Biochemical analysis revealed that the novel small molecules inhibited ENOX1, a plasma membrane- associated enzyme that exhibits protein disulfide-thiol interchange activity. While ENOX activity is important for cellular proliferation, the molecular mechanisms are not well understood. shRNA-mediated inhibition of ENOX activity in HUVECs inhibited endothelial cell migration and formation of tubule-like structures in matrigel, reproducing the effects of the small molecule inhibitors. Small molecule inhibition of ENOX1 was associated with a significant increase in endothelial cell apoptosis induced by ionizing radiation. Colony formation assays demonstrated that these small molecules increased the radiation sensitivity of endothelial cells. In contrast, the radiation sensitivity of tumor epithelial cells was unaffected by the small molecules. Administration of a small molecule ENOX1 inhibitor prior to fractionated X-irradiation produced a statistically significant decrease in the number and density of CD34 expressing Lewis Lung Carcinoma (LLC) tumor-associated microvasculature. Use of LLC and human HT29 colon carcinoma tumor models demonstrated that the small molecule ENOX1 inhibitors coupled with fractionated X-irradiation produced a statistically significant increase in tumor growth delay compared to irradiation alone. No evidence of acute toxicity was observed over a 30 day interval when mice received a bolus injection of 120 mg/kg. These preliminary data support the hypothesis that ENOX1 represents a rationale molecular target for increasing the ability of ionizing radiation to control tumor growth. The short term goal of this application is to valid this hypothesis. The long term goal is to develop these small molecule inhibitors into a radiation sensitizer that exhibits clinical efficacy.